The luminance values in cd/m2 are not very reliable, since camera
sensitivities can easily differ up to 50% from their nominal
value. Typically, the real luminance is 30% lower than this
number. The area used for calculation is chosen such that it
corresponds to a viewing angle of ±14° horizontally and
±10.5° vertically, which is equivalent to viewing the screen
from a distance of about 1.5 times the diagonal size.

Unfortunately, there is a lot of variation in how cameras use EXIF settings.
If photos from your camera your produce unusable results, try coming back in a
few days. I review submitted pictures regularly and try to fix problems.